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The dissolution behaviour of native

International Dairy Journal 16(2006)1427–1434

The dissolution behaviour of native phosphocaseinate as a function of

concentration and temperature using a rheological approach

C.Gaiani a ,J.Scher a ,P.Schuck b ,J.Hardy a ,S.Desobry a ,S.Banon a,?

LSGA,Laboratoire de Sciences et Ge

′nie Alimentaires,2avenue de la Fore ?t de Haye,B.P.172,54505Vandoeuvre Les Nancy Cedex,France b

INRA,UMR Sciences et technologies du lait et de l’?uf,65,rue de Saint-Brieuc,35042Rennes,France

Received 6July 2004;accepted 6December 2005

Abstract

A simpli?ed method to study native phosphocaseinate dissolution was developed.The method involved dispersing powder in distilled water at a constant shear rate.The use of a Stress Tech Rheometer equipped with a custom-built paddle stirrer permitted the determination of the viscosity during the dissolution.The viscosity pro?les obtained highlighted the dissolution stages of the powder including particle wetting,and then swelling followed by a slow disintegration and dissolution of the https://www.doczj.com/doc/681643700.html,ing this method,the viscosities of native phosphocaseinate suspensions were followed as a function of protein concentration,temperature and dissolution time.Indeed,the best knowledge of the dissolution properties could be one way to extend the application of this high protein content powder:the principal limitation of this powder being its poor dissolution ability.r 2006Elsevier Ltd.All rights reserved.

Keywords:Casein;Dissolution;Rheology;Powder

1.Introduction

Most food additives are prepared in powder form and need to be dissolved before use.Water interaction in dehydrated products and dissolution are thus important factors in food development and formulation (Hardy,Scher,&Banon,2002).The dissolution is an essential quality attribute of a dairy powder as a food ingredient (King,1966).Some common concerns are connected to dissolution which depends on different steps:the wett-ability,which is the ability to absorb water;the sinkability which is the ability to sink into the water;the dispersibility,which is the ability to disperse in single particles through-out the water;and the solubility,which is the ability to dissolve in water (Freudig,Hogekamp,&Schubert,1999).There is a continuous need to develop innovative dairy-based ingredients.Native phosphocaseinate (NPC)is one of them;thanks to the developments in membrane ?ltration processing (Fauquant,Maubois,&Pierre,1988).In the present work,we used NPC,which is

obtained by tangential membrane micro?ltration of milk that followed by spray drying of the dia?ltrated retentate (Schuck et al.,1994).This powder constitutes an attractive material due to its high protein content and can be seen as a relevant model of milk micelles (Famelart,Le Graet,&Raulot,1999).Dissolution properties of dairy powders were investigated by Fitzpatrick,Weidendorfer,and Teunou (2000),Fitzpatrick,Weidendorfer,Weber,and Teunou (2001),Schubert (1993),Baldwin and Sanderson (1973);but the dissolution process of NPC was studied less frequently (Gaiani,Banon,Scher,Schuck,&Hardy,2005;Schuck et al.,2002).During rehydration,water transfer of NPC is low in comparison with water transfer of low heat milk powder (Schuck et al.,1994).More information about the dissolution mechanisms can make NPC applications more effective,and this powder could become an attractive material for the food industry.

The knowledge of the rheological behaviour of dairy powders is required in the food industry.Dissolution kinetics can be studied by a rheological approach as for hydrocolloids or gums (Klein Larsen,Gaserod,&Smids-rod,2003;Kravtchenko,Renoir,Parker,&Brigand,1999;Wang,Ellis,&Ross-Murphy,2002).Except for Ennis,

https://www.doczj.com/doc/681643700.html,/locate/idairyj

?Corresponding author.Tel.:+33383595878;fax:+33383595804.

E-mail address:Sylvie.Banon@ensaia.inpl-nancy.fr (S.Banon).

O’Sullivan,and Mulvihill(1998),the use of a rheological approach to study dissolution of dairy-based products has not been reported.

The aims of the present work were to acquire a better understanding of dissolution properties of high protein milk powders.For this purpose,(i)the rheological behaviour of rehydrated NPC solutions was investigated with a conventional geometry,and compared to the paddle geometry to validate the device,(ii)viscosity pro?les obtained during dissolution at different concentrations and temperatures have been interpreted,and(iii)NPC dissolution was followed as a function of technological constants(i.e.protein concentration,temperature,time).

2.Materials and methods

2.1.Powder preparation

NPC was prepared according to Pierre,Fauquant,Le Graet,Piot,and Maubois(1992)and Schuck et al.(1994). This was supplied by Unite Mixte de Recherche(UMR) Sciences et technologie du lait et de l’?uf(INRA,Rennes, France).NPC was separated from skimmed milk by tangential membrane micro?ltration followed by puri?ca-tion through water dia?ltration.

The spray drying of concentrates was performed at Bionov(Rennes,France)in a three-stage pilot-plant spray dryer(GEA,Niro Atomizer,St Quentin en Yvelines, France).The temperature of the product before drying was around40721C.The atomizer was equipped with a pressure nozzle(0.73mm diameter ori?ce)and a4-slot core(0.51mm nominal width),providing a601spray angle. The pressure at the nozzle was16MPa.Inlet temperature was215751C,integrated?uid bed air temperature was 70711C and outlet temperature was70711C.Inlet air humidity was controlled and adjusted by a dehumidi?er (Munters,Sollentuna,Sweden).

2.2.Chemical analysis

The chemical composition of the powder is reported in Table1.The amount of moisture was determined by weight loss after drying1g sample of powder at1051C for5h. Total protein was determined by Kjeldhal with a 6.38 conversion https://www.doczj.com/doc/681643700.html,ctose was determined by enzymatic method using an Enzytec Lactose/D-Galactose kit and fat according to the Ro se Gottlieb method by the IDF(1987). Ashes were measured after incineration at5501C for5h.

2.3.Measuring system for the rheological behaviour of NPC rehydrated solutions:conventional geometry

Triplicate samples of NPC solutions were examined after 12h rehydration at room temperature at5%,8%,10%and 12%;at15,24and551C in a Stress Tech Rheometer (Reologica AB,Lund,Sweden)using a cone and plate geometry(volume sample1.7mL,cone angle41,diameter 40mm).The rheometer was connected to a thermostati-cally controlled bath,and a lid was added onto the sample to prevent evaporation at high temperatures.Samples were allowed to equilibrate at the desired temperature and were subjected to a programmed shear rate increasing from100 to800sà1.

2.4.Measuring system to obtain the viscosity pro?les during NPC dissolution:paddle geometry

The Stress Tech Rheometer was equipped with a custom-built paddle stirrer and a C25cup.The paddle was constructed from four blades specially designed by Rheologica for large particles.The blades were placed at right angles to each other to allow a good homogenization, as shown in Fig.1.Dissolution processes in the industry usually include stirring at constant speed,so the experi-ments were determined at constant shear rate(100sà1). Data were collected automatically every20s during 5000s and then every1000s.All runs were carried out at least in duplicate.

NPC powder was added manually into the rheometer cup.The aqueous phase was distilled water,used as a volume of18mL.The powder was dispersed in the

Table1

Chemical composition of the native phosphocaseinate powder used(mean of three determinations7SD)

g100gà1

Protein86.970.5

Fat0.370.1

Lactose0.470

Ash7.670.2

Moisture 4.87

0.1

fill level

water jacket

C25 cup

Stress-Tech Rheometer head

four blade paddle

stirrer

Fig.1.Apparatus used for monitoring rheological changes during native phosphocaseinate powder dissolution in distilled water.

C.Gaiani et al./International Dairy Journal16(2006)1427–1434 1428

rheometer cup,50s after starting the rheometer.The dissolution is strongly dependent on temperature and concentration.To study these effects,six levels of total nitrogen concentration were employed as:2%,5%,8%, 9%,10%and12%(w/v).For each preparation,0.2%(w/v) of sodium azide was added to the distilled water. Temperature effects were evaluated at six levels as:5,15, 24,35,45and551C.

2.5.Static light scattering

The particle size distributions were measured from a laser light diffraction apparatus(Mastersizer S,Malvern Instruments Ltd,Malvern,UK)with a5mW He–Ne laser operating at a wavelength of632.8nm with a300RF lens.

2.5.1.NPC particle

The particle size distribution of dried particles was determined using a dry powder feeder attachment,and the standard optical model presentation for particles dispersed in air was used.

2.5.2.NPC suspension

From the rheometer cup,0.5mL of NPC suspension were taken and introduced in100mL of pre-?ltered distilled water(Millipore,membrane diameter0.22m m)to reach a correct obscuration.The Malvern small volume sample cell used permitted the maintainence of a stable suspension during the measurement under a2000rpm stirring.The refractive indices used were1.57for casein and1.33for water(Strawbridge,Ray,Hallett,Tosh,& Dalgleish,1995).The results obtained are average dia-meters calculated from Mie theory.The criterion selected was d(50)that means that50%of the particles have a diameter lower than this criterion(i.e.midpoint of volume cumulative distribution).Results are the average of triplicate experiments carried out on different days.

2.6.Microscopy images

A phase contrast microscope(Leica DMRB,Wetzlar, Germany)was used to monitor NPC particle dissolution.It was connected to a CCD video camera(Kappa optoelec-tronics GmbH,Germany)controlled by an image proces-sor(Kappa imageBase2.5).

3.Results and discussion

3.1.Rheological behaviour of NPC suspension:validation of the paddle geometry

The?ow curves of NPC solutions after12h rehydration are displayed at5%,8%,10%and12%(w/v)NPC at15, 24and551C.The rheological behaviour of all the solutions at all the temperatures was Newtonian with a R2close to 0.99.All the viscosities reported increased with increasing concentration and decreased with increasing temperature.Brie?y,the viscosities were found in the range of 2.5–4.6mPa s at5%(15–551C),4–5.9mPa s at8%, 4.6–7.5mPa s at10%and6.5–8.5mPa s at12%.Bourriot, Garnier,and Doublier(1999)reported similar Newtonian behaviour for solutions at3%NPC;201C and after5h dissolution.

The values obtained for2%,5%,8%,10%and12% NPC at15,24and551C with the paddle and the conventional geometry are presented in Fig.2.They were in good agreement.In brief,at241C and5%(Fig.2B),the apparent viscosity obtained after12h in the rheometer cup was very close to the apparent viscosity of12h-rehydrated NPC solution obtained with the cone and plate geometry; these were respectively of 4.1and 3.7mPa s.A slight dehydration of the solution occurring in the rheometer cup could explain the minor overestimation of viscosity with the paddle geometry.The comparisons between a conven-tional geometry(cone and plate)and our system(paddle) allowed us to validate the values reported in this work with the paddle.

3.2.Viscosity typical pro?le during NPC powder dissolution 3.2.1.Interpretation of viscosity and particle size pro?les The dissolution of5%NPC at241C occurred in different stages as shown in Fig.3.Dispersion of powder in the rheometer cup led to a quick increase of viscosity (peak a).This was followed by an other increase of viscosity with a maximum after about2000s(peak b).Next to peak b,a decrease in viscosity occurred(phase c).At the end of the pro?le,a low viscosity?uid is formed and the viscosity value is stabilized at

4.1mPa s around48,000s (phase d).

During5%NPC dissolution,samples were taken into the rheometer cup and the particle size was determined by static light scattering.As shown in Fig.3,the events observed by viscosity measurements(peak b,phase c and d)were related to particle size variations.Dispersion of powder in the rheometer cup led to a quick increase of viscosity(peak a)certainly due to particles wetting.The wetting stage was followed by swelling of the particles from 286to386m m corresponding to the peak b in viscosity.As a consequence of the swelling,a disintegration of the wet particles and their progressive dissolution(phase c)could explain viscosity and particle size decrease.Around 48,000s,the particle size(0.36m m)and viscosity stabiliza-tion were observed probably due to the end of dissolution (phase d).

In Fig.4,microscopy images of NPC dissolution permitted to visualize the swelling(peak b)and dissolution phenomena(phase c).Moreover,the size of the dry particle was found around250m m and con?rmed the static light scattering results(Fig.4A).The same particle after wetting started to swell from250to400m m(Fig.4B).After the swelling,a slow dispersion of the particle occurred(Fig.3C and D).These observations were in agreement with the interpretation of Fig.3;and also with the results found by

C.Gaiani et al./International Dairy Journal16(2006)1427–14341429

Gaiani et al.(2005),where they described the rehydration of NPC powder by turbidity measurements.

3.2.2.Proposition of a suitable model of NPC dissolution From the viscosity pro?le and the particle size distribu-tions obtained,a general model of NPC dissolution was proposed in Fig.5.NPC dissolution occurred in different

stages as:wetting and swelling followed by a slow disintegration and dissolution of the particles to ?nally reach a ?uid in appearance similar to milk.The dissolution process clearly takes time to achieve a stable dispersion con?rmed by Schuck et al.(1994,2002)and Gaiani et al.(2005).At the end of NPC dissolution,we obtained a particle size of 0.36m m (70.04)corresponding to casein micelles in agreement with Regnault,Thiebault,Dumay,and Cheftel (2004).

3.3.Viscosity pro?les during NPC dissolution

3.3.1.Effect of NPC concentration

At low concentration (2%)in Fig.6A ,only the wetting peak (a)was noticeable.No swelling peak was observed.The use of static light scattering con?rmed that the swelling phenomena appeared even at 2%NPC.The viscosity pro?les at low protein content could be explained by rheometer limitation.

At 5%NPC,the viscosity pro?le obtained (Fig.6B )correspond to the typical pro?le (Fig.3);powder wetting (peak a),powder swelling (peak b)around 2000s that is followed by a slow dissolution of the particles (phase c)to obtain a rehydrated NPC solution after 48,000s.

At 12%and 10%NPC,three peaks appeared on the viscosity pro?le (Fig.6C and D ).The concentration in?uence on the viscosity pro?le was unexpected.The ?rst peak (peak a)corresponds to powder wetting.The peaks b 0and b 00are supposed to correspond to powder swelling.Phases c 0and c are related to powder disintegration.The two peaks of viscosity b 0and b 00,noticeable from 10%protein content,were supposed to reveal two swelling stages.The ?rst one could correspond to the swelling of the initial particles,and then the second one to the swelling of newly free particles in the aqueous media.This hypothesis could explain the delay in the second peak of swelling (b 00)

024681002468100246810

(A)

(B)(C)

V i s c o s i t y c o n e /p l a t e (m P a .s )

410

Viscosity paddle (mPa.s)

Fig.2.Viscosity values obtained for 2%,5%,8%,10%and 12%native phosphocaseinate;at 151C (A),241C (B)and 551C (C)with the paddle and cone/plate geometry (mean of four determinations 7SD).

100100010000

100000

10100

V i s c o s i t y (m P a .s )

0.1

0.01

0.001

P a r t i c l e s i z e (μm )

10000.1Time (s)

Fig.3.Viscosity typical pro?le and particle size shown as a function of time obtained during dissolution of 5%native phosphocaseinate;at 241C for 50,000s.,b

ing to t

with increasing protein content.Indeed,bulk water availability is supposed to diminish with increasing protein concentration leading to the slowing down of the subsequent stages of protein rehydration.

3.3.2.Effect of temperature

Viscosity pro?les during dissolution were measured at 5%at 5,15,24and 451C.For each temperature studied,the wetting peak (peak a)was present.At low temperatures (Fig.7A and B ),the swelling stage was very slow and the time to reach maximum viscosity (peak b)was late.When the temperature of dissolution increased,the time to reach maximum viscosity was shortened and the swelling stage appeared quicker (Fig.7C ).At higher temperatures,the swelling of the particles was so quick that it was impossible to determine the time precisely (Fig.7D ).Indeed,the peak b is probably mixed up with the peak a (peak ab).

3.4.Following the NPC dissolution as a function of temperature,concentration and time

We computed all the viscosities data in Fig.8to follow NPC dissolution as a function of temperature,concentra-tion and dissolution time.For example,the dissolution of 2%NPC (Fig.8A )is completed after 50,000s from 15to 551C.Indeed,the viscosity at 50,000s dissolution is the same as the viscosity of the reference rehydrated solution.At 51C,the viscosity obtained at 50,000s was higher than the viscosity of the reference rehydrated solution,meaning that the dissolution was not achieved.After 5000and

Fig.4.Optic microscopy performed on native phosphocaseinate particle during powder dissolution.(A)dried particle,(B)particle after 2000s dissolution,(C)particle after 20,000s dissolution and (D)suspension at the end of dissolution.

V i s c o s i t y (m P a .s )

Time (s)

Fig.5.A suitable model proposed of native phosphocaseinate dissolution based on viscosity pro?les and particles size distributions.,b

ng to the

10,000s,we observed that dissolution of 2%NPC is completed only from 35to 551C.A decrease in tempera-ture slowed down the reactions of dissolution which became longer.In practical situations,this could permit development of optimal time–energy conditions to dissolve NPC powder.

1001100011010

10010001

100100010000

10000

1101001000V i s c o s i t y (m P a .s )V i s c o s i t y (m P a .s )Time (s)

Time (s)

(A)(C)

(D)

Fig.6.Viscosity pro?les obtained during dissolution of native phosphocaseinate at different concentrations;at 241C for 10,000s.(A)2%,(B)5%,(C)

10%and (D)12%,

b the secon

110

100Time (s)

Time (s)

1001

100

10000

100

10000

1(A)

(C)

(D)

V i s c o s i t y (m P a .s )

Fig.7.Viscosity pro?le obtained during dissolution of 5%native phosphocaseinate at different temperatures for 50,000s.(A)51C,(B)151C,(C)241C and (D)451C,,b

d peak o

If the NPC concentration increased (Figs.8B–D ),the complete dissolution is hardly reached even for 50,000s dissolution.For example,after 50,000s,NPC dissolution at 5%,10%and 12%is completed by increasing temperature at 24,35and 451C,respectively.Over 451C,the time to rehydrate the powder was only slightly improved and attributed to casein precipitation or paste formation outside of the particle.

4.Conclusions

The viscosities obtained were in good agreement with the conventional geometry and the method showed good reproducibility.The use of a Stress Tech Rheometer equipped with a paddle stirrer to follow viscosity during NPC dissolution was validated in this study.

The present results gave indications about the mechan-isms involved during NPC dissolution at constant stirring and the analysis of viscosity pro?les seems to be a promising tool to understand powder dissolution stages.We showed that temperature,concentration and time were the major factors in?uencing NPC dissolution.It is also possible to improve the dissolution by changing the chemical composition or the method of manufacture and storage conditions of the powders;these factors being closely connected with the physical structure of the powder.Further studies applying this approach to NPC powders are in progress.

Acknowledgements

The authors are indebted to ARILAIT RECHERCHES for numerous simulating discussions and ?nancial support.

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